I think indifferent does not apply to her emotional state but to her health. Although this is seems a strange word to choose according to its modern interpretation. It can imply neutrality--neither good or bad--rather than being ill as is suggested in the earlier paragraph. The Merriam-Webser online dictionary also suggeests apathy, mediocrity or disinterest. So perhaps Mr Collins is trying to suggest that Miss de Bourgh's state of health fluctuates such that it prevents her from being in town.